As a control method for an internal combustion engine, torque demand control for determining operation amounts of actuators using a torque as a controlled variable is known. The actuators operated through torque demand control include those regarding the amount of air, those regarding the ignition timing, and those regarding the air-fuel ratio. Among these actuators, those regarding the amount of air include, for example, a throttle, a variable valve timing mechanism that changes the valve timing of an intake valve, and a variable valve timing mechanism that changes the valve timing of an exhaust valve.
FIG. 7 is a functional block diagram showing the configuration of a control apparatus for an NA engine that performs conventionally proposed torque demand control. A control apparatus 200 shown in FIG. 7 is designed to operate a throttle 10, an intake valve variable valve timing mechanism (hereinafter referred to as an IN-VVT) 20, and an exhaust valve variable valve timing mechanism (hereinafter referred to as an EX-VVT) 30. Then, the control apparatus 200 is equipped with a target air amount calculation unit 210, a VVT control unit 220, and a throttle control unit 230.
The target air amount calculation unit 210 calculates an amount of air needed to realize a required torque, as a target air amount. The calculation is carried out with the aid of a map in which a torque and an air amount are associated with each other using various pieces of engine information such as an engine rotational speed, an ignition timing, an air-fuel ratio and the like as arguments.
The VVT control unit 220 selects a combination that optimizes fuel economy, among combinations of a valve timing of an intake valve that can be realized through operation of the IN-VVT 20 and a valve timing of an exhaust valve that can be realized through operation of the EX-VVT 30. Such a combination is stored in advance as a base valve timing. The VVT control unit 220 determines a command value for the IN-VVT 20 (an IN-VVT command value) and a command value for the EX-VVT 30 (an EX-VVT command value) respectively, in accordance with the base valve timing.
Besides, the VVT control unit 220 stores, in the form of a map, a relationship that is established among a valve overlap amount, an intake pressure, and an air amount. In FIG. 7, an image of the map is represented in the form of a graph in a block indicating the VVT control unit 220. The air amount that is associated with the valve overlap amount and the intake pressure in this map is, in a precise sense, an amount of air that has entered a cylinder through the intake valve. On the other hand, the target air amount that is calculated from the required torque is, in a precise sense, an amount of air used for combustion, namely, a target value of an in-cylinder air amount. However, as will be described later, an intake valve passing air amount coincides with the in-cylinder air amount at least in an NA engine, so there is no problem in applying the target air amount to the aforementioned map. The VVT control unit 220 selects an intake pressure corresponding to the valve overlap amount at the base valve timing, among combinations of the intake pressure and the valve overlap amount that can realize the target air amount, and determines the selected intake pressure as a target intake pressure.
The throttle control unit 230 calculates a throttle opening degree from the target intake pressure and the target air amount. The inverse model of an air model is used to calculate the throttle opening degree. The air model is a physical model that is obtained by modeling dynamic properties of the pressure and flow rate in an intake passage in response to the motion of the throttle. The throttle control unit 230 operates the throttle 10 using the calculated throttle opening degree as an operation, amount.
The control apparatus thus configured makes it possible to control the in-cylinder air amount in the engine to an air amount that is neither too large nor too small to realize the required torque, through cooperative operation of the throttle 10, the IN-VVT 20, and the EX-VVT 30.
By the way, it is conceivable to apply the aforementioned torque demand control to the control of a supercharged engine that is equipped with a turbosupercharger or a mechanical supercharger. If the configuration for torque demand control of the NA engine shown in FIG. 7 is directly utilized, a configuration shown in, for example, FIG. 8 can be adopted as a configuration of the control apparatus that is needed to perform torque demand control of the supercharged engine. A control apparatus 201 shown in FIG. 8 is designed to operate a waste gate valve (hereinafter referred to as a WGV) 40 in addition to the throttle 10, the IN-VVT 20, and the EX-VVT 30. Then, the control apparatus 201 is equipped with a target supercharging pressure calculation unit 240 and a WGV control unit 250 in addition to the target air amount calculation unit 210, the VVT control unit 220, and the throttle control unit 230.
In the case of a supercharged engine, the intake pressure may reach an upper limit due to the opening of the throttle 10 to a fully open state in a supercharging region where supercharging is carried out by a supercharger. In that case, the VVT control unit 220 specifies combinations of the intake pressure and the valve overlap amount that can realize the target air amount with the aid of the foregoing map, and selects the valve overlap amount corresponding to the upper limit of the intake pressure out of those combinations. It should be noted, however, that there are many combinations of the valve timings of the intake valve and the exhaust valve that can realize the selected valve overlap amount, so the command values for the respective variable valve timing mechanisms 20 and 30 are not uniquely determined. For instance, it is conceivable to select the combination closest to the base valve timing, and determine the command values for the respective variable valve timing mechanisms 20 and 30 in accordance with the selected combination. Incidentally, in the case where the intake pressure has not reached the upper limit, the base valve timing is selected as a combination of the respective valve timings of the intake valve and the exhaust valve, and the intake pressure corresponding to the valve overlap amount at the base valve timing is determined as the target intake pressure, as is the case with the NA engine.
The target supercharging pressure calculation unit 240 calculates a value obtained by adding a predetermined reserve pressure to the target intake pressure, as a target supercharging pressure. The WGV control unit 250 determines a duty value imparted to a solenoid that drives the WGV 40, based on the target supercharging pressure. As an example of the method of determining the duty value, it is possible to mention a method in which a map that associates the duty value with the supercharging pressure is prepared and a duty value corresponding to the target supercharging pressure is calculated from the map. Besides, it is also possible to mention a method in which an actual supercharging pressure is measured or estimated, and the duty value is subjected to feedback control such that the actual supercharging pressure becomes equal to the target supercharging pressure.
The control apparatus thus configured makes it possible to cause the engine to output a required torque through the same control as in the case of the NA engine, in an NA region where supercharging is not carried out by the turbosupercharger.
However, in the supercharging region where supercharging is carried out by the turbosupercharger, the following problem arises as to the accuracy in realizing the required torque.
FIG. 9 shows an image of a control result in the supercharging region by the control apparatus configured as shown in FIG. 8. According to the control apparatus shown in FIG. 8, the target air amount is calculated from the required torque, and the throttle 10, the IN-VVT 20, the EX-VVT 30, and the WGV 40 are cooperatively operated in such a manner as to realize the target air amount. However, the air amount that is actually realized by the control apparatus shown in FIG. 8 is smaller than the target air amount.
This shortfall in the air amount results from direct application of the control method for the NA engine to the supercharged engine. In the NA engine, the exhaust pressure is higher than the intake pressure. Therefore, in the case where the open period of the intake valve and the open period of the exhaust valve overlap with each other, combustion gas remains in the cylinder in accordance with the overlap amount. In other words, so-called internal EGR is caused. In this case, the air (fresh air) that has entered the cylinder through the intake valve remains in the cylinder, and the sum of the intake valve passing air amount and the amount of residual combustion gas resulting from internal EGR is a total amount of gas in the cylinder. Consequently, in the case of the NA engine, the intake valve passing air amount and the amount of air in the cylinder that is actually used for combustion coincide with each other, regardless of whether or not those open periods overlap with each other.
On the other hand, in the case of the supercharged engine, the intake pressure is higher than the exhaust pressure in the supercharging region. Therefore, in the case where the open period of the intake valve and the open period of the exhaust valve overlap with each other, scavenging, namely, the blow of air from an intake pipe through an exhaust pipe occurs in the supercharging region. In the case where scavenging occurs, part of the air that has entered the cylinder through the intake valve flows through the exhaust pipe. Therefore, the in-cylinder air amount that is actually used for combustion is smaller than the intake valve passing air amount by an amount corresponding to scavenging. As a result, as shown in FIG. 9, the in-cylinder air amount that is actually realized is smaller than the target air amount, so the realized torque falls short of the required torque.
As is apparent from the foregoing, in the case where torque demand control is applied to the supercharged engine, it is inappropriate to directly utilize the configuration for torque demand control in the NA engine. Torque demand control for the supercharged engine requires a control apparatus that makes it possible to accurately realize the required torque even in the supercharging region where scavenging occurs.
Incidentally, the documents mentioned below are related-art documents indicating the state of the art in a technical field according to the present application.